Another component. Fun, right? Well this component is absolutely necessary to circuitry. Without it, almost every circuit out there would become useless if it suddenly disappeared. So, now that you're thinking, you're probably wondering what it does. Well, simple! Transistors are like an automated switch. It's almost like some of the switches we saw a few posts back, except you're essentially having a current of electricity to flip the switch rather than your finger, making it, in my opinion, one of these some of the most useful components out there.
Transistors are actually categorized into two main categories: NPN and PNP. This stands for "negative positive negative" and PNP stands for "positive negative positive." They're basically opposites. Why is it named that? Well, look at the picture above. If you look at the bottom right picture, you'll see that rectangle is split into three portions: the collector, base, and emitter. The collector is like the first lead in a SPST switch, the base like the toggle, and the emitter like the other lead of the switch. If you were to use one for switching, you would basically have the collector hooked up to a positive source, the emitter to your ground, and the base to some sort of signal or pulse. When you apply power to the collector, practically no current flows through when the base doesn't receive a signal. When the base does receive a signal, the current flows from collector to emitter without a problem. This is where the difference between PNP and NPN comes in. The one above is an NPN transistor because positive voltage flows from collector to emitter and uses a positive voltage to trigger the base. The only difference in a PNP transistor is a polarity swap. Instead of positive voltage going in through the collector, it goes in through the emitter and grounds through the base.
So how do these work? No, there's no black magic going on here, it's actually pretty simple. The transistor is composed of three parts, collector base and emitter, that are sandwiched together. All three of these parts are made of silicon, which is a great semiconductor. What's a semiconductor? It's something that conducts electricity with an efficiency between that of an insulator and metal. But there is a difference between the three sections. In an NPN, the two pieces of bread, collector and emitter, are negatively doped. Yes, negatively doped, meaning the silicon is infused with an element with more electrons making it more negative, like phosphorus. Now the base is the opposite, it's positively doped, meaning it's infused with an element with one less valence electron than silicon, like the element boron. Because the base is positively charged and the two other pieces are negatively charged, the positive base actually steals electrons from the negative sides to fill equalize everything out. The result of this is actually a little barrier-type deal that forms where the bread meets the meat where the base is actually slightly negative, repelling electrons attempting to flow from one end to the other. This barrier is very small and relatively weak, making it easy to break letting current flow through. How do we do this? This where the small positive current from the base comes in. If you look at the picture, you'll see a small plate hovering above the meat. When the plate is positively charged, it attracts the electrons from the collector and pulls it over to the meat. This process actually widdles the barrier down to practically nothing, giving the electrons a straight, undisturbed path from collector to emitter. It's amazing, right?
Now that we know how they work, let's apply them. One very basic NPN transistor that is used almost everywhere is the 2N2222 transistor. This one works like any other NPN transistor. There are a few rules we need to follow when using them. There needs to be a level of voltage applied to each piece of the sandwich relative to the other pins. This means that the collector needs to be the most positive, the emitter the least positive, and the base somewhere in between. So in this circuit, you have nine volts reduced by 560 ohms leading to the collector. Since the collector is reduced by more the 560 ohms for it to work, you'd have to use a resistor more resistive than 560. In this circuit, they actually replaced the resistor with your finger. Your finger is pretty stinking resistant than 560 ohms; depending on how far the two wires are separated. I clocked in about 800 ohms with a half inch of Dylan finger with my multimeter. Since the base has a weaker current being supplied to it, the base's current can flow through; resulting in an even weaker output from the emitter. Every thing checks out! So, when you touch the two wires to your finger, the current from the base can flow through, completing the circuit and in turn, illuminating the LED.
I've only scraped the surface here. There are so many different types of transistors out there that it'll take you a week to learn the fundamentals to each one. There are MOSFETs, JFETs, IGBT, bipolar, junction, Darlington; it's nuts. Here's a Youtube video that will hopefully make more sense to you. I can only do so much here on a blog! The best way to learn, and it's the way I've learned, is online videos, books, and trial and error. Hopefully here pretty soon I'll provide a list of good sources and books you can look at.
